Preparation method of tungsten carbide sintered body for friction stir welding tool

ABSTRACT

The present invention relates to a preparation method of a tungsten carbide sintered body for a friction stir welding tool used in a friction stir welding tool of a high melting point material such as steel, titanium and the like or a dissimilar material such as aluminum, magnesium-steel, titanium and the like using pulsed current activation through a discharge plasma sintering apparatus. The preparation method comprises the following steps: filling a tungsten carbide (WC) powder in a mold made of a graphite material; mounting the mold filled with tungsten carbide powder in a chamber of a discharge plasma sintering apparatus; making a vacuum inside of the chamber; molding the tungsten carbide powder while maintaining a constant pressure inside the mold and increasing the temperature according to a set heat increase pattern until the temperature reaches a final target temperature; and cooling the inside of the chamber while maintaining the pressure pressurized in the mold after the molding step. According to the preparation method of a tungsten carbide sintered body for a friction stir welding tool, it is possible to obtain a high relative density of 99.5% or higher, and to prepare a uniform sintered body having a homogeneous tissue with little particle growth, high toughness, high abrasion resistance and high strength within a short time by a single process when preparing a tungsten carbide sintered body appropriate for a friction stir welding tool by using pulsed current activation through a discharge plasma sintering apparatus. In addition, since a sintered body is prepared with only a tungsten carbide single material, excluding a sintering additive such as cobalt, a preparation method is simplified, preparation costs are reduced, and toughness, abrasion resistance and strength are superior compared with a sintered body containing cobalt, a sintering additive.

TECHNICAL FIELD

The present invention relates to a preparation method of a tungstencarbide sintered body for a friction stir welding tool, and moreparticularly, to a preparation method of a tungsten carbide sinteredbody for a friction stir welding tool, which can prepare a uniformsintered body having high density, and high strength, high toughness andhigh abrasion resistance, while having little difference betweeninternal and external physical properties, within a short time by asingle process through a discharge plasma sintering process.

BACKGROUND ART

From the point of view of energy saving and environmental protection, alightweight technology for various transportation means, such asautomobiles, aircrafts, railway vehicles and ships, has emerged. In awelding process of lightweight materials, a friction stir welding (FSW)technology which is a non-melting solid state welding technology isapplied. Recently, the FSW technology spotlighted as a next generationtechnology is widely used in welding of similar and dissimilar materialsof high melting point materials, such as titanium, steel, stainless andnickel alloys, as well as the lightweight materials, and also applied tovarious industrial fields.

In order to weld the high melting point materials, it is necessary todevelop a tool material having a long lifespan, and thus variousmaterials are developed and studied to satisfy high strength, highabrasion resistance, high toughness, uniformity of micro-structures andthe like. For example, as this kind of material, there are PCBN preparedby MegaStir company located in U.S.A., and Ir—W, Ir—Re, Ir—Mo or thelike prepared by Furuya company located in Japan. However, the toolsprepared in U.S.A. and Japan have excellent strength and toughness, buthave disadvantages of a high price and a short lifespan.

Tungsten carbide (WC) has a melting point of 2600° C., and a density of15.7 g/cm³, and cobalt (Co) has a melting point of 1459° C., and adensity of 8.9 g/cm³. WC—Co is called as cemented carbide, hasadvantages in ceramics and metals and thus is applicable for variouspurposes. Since the tungsten carbide has a high melting point, highstrength and high abrasion resistance, it is used for various purposes,such as machining tools, abrasion resistive tools, cutting tools andmolds, and when the Co is added, toughness is improved, and thus it ispossible to prepare a material having high toughness.

Recently, a preparation method of the WC-Co which is spotlighted as amaterial for a solid state friction stir welding tool can be classifiedinto a melting/casting method, and a powder metallurgy method. Themelting/casting method is the most general methods of sintering andpreparing the WC—Co, which have some advantages of facilitating massproduction and reducing preparation costs, but also have somedisadvantages of limiting control of fine particles and high density.Furthermore, the above methods require several post-treatment processes.

However, in case of using the powder metallurgy method, since it hasadvantages of uniform phase distribution, control of fine particles andfacile preparation of a high melting point material, and also has anadvantage which can prepares the WC-Co having the high toughness andhigh strength, it is widely used as a substitute process for themelting/casting method.

However, as conventional powder metallurgy methods, a hot isostaticprocessing (HIP) method and a hot pressing (HP) method which can obtaina sintered body having a relative high density by simultaneouslyapplying temperature and pressure have been mainly used, but developmentof a new process technology is required due to deterioration ofstrength, toughness and abrasion resistance caused by a long moldingprocess time and thus limitation of fine particle control, and adifference between internal and external physical properties and anexpensive process cost caused by an external heating system.

To satisfy the requirement, the applicant has filed a patent applicationfor a preparation method of a WC—Co sintered body, which can be used forthe solid state friction stir welding tool, using a discharge plasmasintering process.

However, in the WC—Co sintered body, since it is necessary to add Co toWC, additional processes are required over multiple stages in thepreparation method. When preparing the sintered body, a process cost isincreased and hardness thereof is reduced due to the addition of the Co,and thus it is restricted to be used as the high melting point frictionstir welding tool due to a short lifespan. Therefore, a new method whichcan prepare a sintered body for the solid state friction stir weldingtool using only the WC without a sintering additive such as Co isrequired.

DISCLOSURE OF THE INVENTION

In order to overcome the above-mentioned shortcomings, the presentinvention provides a preparation method of a tungsten carbide sinteredbody for a friction stir welding tool, which can prepare a high meltingpoint uniform sintered body having a homogeneous structure with highdensity, high strength, high toughness and high abrasion resistancewithin a short time by a single process using pulsed current activationthrough a discharge plasma sintering apparatus, while can controlparticle growth of the tungsten carbide sintered body for the solidstate friction stir welding tool, and also which has a lower processcost, compared with the HP method or the HIP method, and has littledifference between internal and external physical properties.

According to an aspect of the invention, there is provided a preparationmethod of a tungsten carbide sintered body for a friction stir weldingtool, including filling tungsten carbide (WC) powder in a mold made of agraphite material; installing the mold filled with the WC powder in achamber of a discharge plasma sintering apparatus; making a vacuuminside the chamber; molding the WC powder in the mold while maintaininga constant pressure inside the mold and increasing a temperatureaccording to a set heating pattern until the temperature reaches a finaltarget temperature; and cooling the inside of the chamber whilemaintaining the pressure applied in the mold after the molding.

The final target temperature of the molding may be 1410 to 2000° C.

The WC powder in the filling may have a particle size of 10 to 10011 m,and the preparation method may further include preliminarilypressurizing the WC powder at a pressure of 1400 to 1600 kgf andmaintaining the pressure for 5 to 15 minutes, after the filling of theWC powder in the mold.

The molding may maintain the WC powder filled in the mold at a pressureof 30 to 100 MPa, and the molding may include primarily heating the WCpowder in the mold to a first target temperature at a heating rate of 60to 150 ° C./min; maintaining the first target temperature for 1 to 10minutes; secondarily heating the WC powder in the mold to a secondtarget temperature at a heating rate of 30 to 80° C./min; maintainingthe second target temperature for 1 to 10 minutes; thirdly heating theWC powder in the mold to a third target temperature at a heating rate of10 to 80° C./min; maintaining the third target temperature for 1 to 10minutes; fourthly heating the WC powder in the mold to a fourth targettemperature at a heating rate of 10 to 80° C./min; maintaining thefourth target temperature for 1 to 10 minutes; fifthly heating the WCpowder in the mold to a fifth target temperature at a heating rate of 10to 80 ° C./min; maintaining the fifth target temperature for 1 to 10minutes; sixthly heating the WC powder in the mold to a sixth targettemperature at a heating rate of 10 to 80° C./min; maintaining the sixthtarget temperature for 1 to 10 minutes; seventhly heating the WC powderin the mold to a seventh final target temperature at a heating rate of10 to 80 ° C./min; and maintaining the seventh final target temperaturefor 1 to 10 minutes, and the first target temperature may be 550 to 650°C., the second target temperature may be 900 to 1005° C., the thirdtarget temperature may be 1010 to 1105° C., the fourth targettemperature may be 1110 to 1205° C., the fifth target temperature may be1210 to 1305° C., the sixth target temperature may be 1310 to 1405° C.,and the seventh final target temperature may be 1410 to 2000° C.

The WC sintered body has a relative density of 99.5% or more.

According to the preparation method of the tungsten carbide sinteredbody for the friction stir welding tool of the present invention, it ispossible to obtain a high relative density of 99.5% or higher, and alsoto prepare the uniform sintered body having a homogeneous structure withlittle particle growth, high toughness, high abrasion resistance andhigh strength within a short time by a single process by using pulsedcurrent activation through a discharge plasma sintering apparatus, whilehaving little difference between internal and external physicalproperties and a thick thickness and large surface area. In addition,since the sintered body is prepared with only the tungsten carbidesingle material, excluding a sintering additive such as cobalt, thepreparation method is simplified, preparation costs are reduced, andtoughness, abrasion resistance and strength thereof are superiorcompared with a sintered body containing cobalt as the sinteringadditive.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will bemore apparent from the following detailed description in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a view schematically illustrating a discharge plasma sinteringapparatus used in a preparation method of a tungsten carbide sinteredbody for a friction stir welding tool in accordance with the presentinvention;

FIG. 2 is a photograph taken through a scanning electron microscope oftungsten carbide powder before a sintering process which is applied inthe preparation method of the tungsten carbide sintered body for thefriction stir welding tool in accordance with the present invention;

FIG. 3 is a graph illustrating a result of XRD component analysis of thetungsten carbide powder before the sintering process applied to FIG. 2;

FIG. 4 is a photograph taken through a scanning electron microscope of atungsten carbide sintered body which is prepared in accordance with thepresent invention and of which a surface is polished and then etched bya murakami etching process;

FIGS. 5 and 6 are photographs of tungsten carbide sintered bodies havinga relative density of 99.8% or more, a diameter of 65.5 mm and athickness of 30 mm, which are prepared at a pressure of 70 MPa, atemperature of 1400 to 2000° C. and a heating rate of 35° C./min;

FIG. 7 is a photograph of a tool before a test, which is prepared bymachining a test piece;

FIG. 8 is a photograph of the tungsten carbide tool of FIG. 7 afterinstalled in an apparatus and performing a friction stir welding testwith respect to a SS400 (a tensile strength of 400 MPa class) steelplate;

FIG. 9 is a photograph of a SS400 (a tensile strength of 400 MPa class)steel plate and a currently available tungsten carbide-cobalt toolbefore and after installed in an apparatus and performing a frictionstir welding test with respect to the steel plate;

FIG. 10 is a photograph of the tungsten carbide tool of FIG. 7 afterperforming a test process of 50 m or more;

FIG. 11 is graph illustrating a change in a weight of the tungstencarbide tool of FIG. 7 while performing the test process of 50 m ormore; and

FIG. 12 is a graph illustrating a result of XRD component analysis withrespect to the sintered body of FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a view schematically illustrating a discharge plasma sinteringapparatus used in a preparation method of a tungsten carbide sinteredbody for a friction stir welding tool in accordance with the presentinvention.

Referring to FIG. 1, a discharge plasma sintering apparatus 100 includesa chamber 110, a cooling part 120, a current supplying part 130, atemperature detecting part 140, a pump 150, a pressurizer 160, a maincontroller 170 and an operating part 180.

An upper electrode 211 and a lower electrode 212 are disposed in thechamber 110 so as to be spaced apart from each other. Although notillustrated, the upper electrode 211 and the lower electrode 212 areconfigured so that cooling water for heat emission can flowtherethrough.

The cooling part 120 is configured so that the cooling water can flowthrough a cooling water flowing pipe provided in a wall of the chamber110 and another cooling water flowing pipe provided in the upperelectrode 211 and the lower electrode 212.

The current supplying part 130 is controlled by the main controller 170so as to supply a pulsed current through the upper electrode 211 and thelower electrode 212.

The temperature detecting part 140 may be formed in an infraredtemperature detecting manner which detects a temperature through a sightglass provided at the chamber 110.

The pump 150 is configured to discharge inner air in the chamber 110 toan outside.

The pressurizer 160 is installed to pressurize tungsten carbide powder205 filled in a mold 200. In the drawing, the pressurizer 160 has acylinder structure which can move up and down a lower portion of thelower electrode 212.

The main controller 170 controls the cooling part 120, the currentsupplying part 130, the pump 150 and the pressurizer 160 according to anoperation command set through the operating part 180, and receivestemperature information detected from the temperature detecting part 140and then displays the temperature information through a displaying part(not shown).

The mold 200 is formed into a cylindrical shape having a receivinggroove formed in a central portion thereof to receive the tungstencarbide powder.

The discharge plasma sintering apparatus 100 may have spacer between themold 200 and the upper and lower electrodes 211 and 212, such that acurrent applied from the upper and lower electrodes 211 and 212 to themold 200 can be concentrated, heating efficiency can be improved, andalso unnecessary energy consumption can be reduced. That is, first tothird upper spacers 221, 222 and 223 made of a graphite material, ofwhich each diameter is gradually reduced toward an upper punch 215, areprovided between the upper electrode 221 for applying an electric fieldinto the mold 200 and the upper punch 215 entering into the mold 200from an upper side of the mold 200. Further, first to third lowerspacers 231, 232 and 233 made of a graphite material, of which eachdiameter is gradually reduced toward a lower punch 216, are providedbetween the lower electrode 221 and the lower punch 216 extended fromthe lower electrode 212 and entering into the mold 200 from a lower sideof the mold 200.

According to such insertion structure of each of the upper and lowerspacers 221, 222, 223, 231, 232 and 233, the current is concentratedfrom the upper and lower electrodes 211 and 212 to the mold 200 throughthe punches 215 and 216, and thus it is possible to increase the currentutilization efficiency and the heating efficiency. The first upperspacer 221 and the first lower spacer 231 may have a diameter of 350 mmand a thickness of 30 mm, the second upper spacer 222 and the secondlower spacer 232 may have a diameter of 300 mm and a thickness of 60 mm,and the third upper spacer 223 and the third lower spacer 233 may have adiameter of 100 to 200 mm and a thickness of 15 to 30 mm.

Hereinafter, processes of preparing a tungsten carbide sintered bodyusing the discharge plasma sintering apparatus 100 will be described.

A preparation method of a tungsten carbide sintered body for a frictionstir welding tool using pulsed current activation through the dischargeplasma sintering apparatus according to the present invention includes afilling process, an installing process, a vacuum-making process, amolding process and a cooling process.

In the filling process, the tungsten carbide (WC) powder to be sinteredis filled in the mold 200 made of the graphite material.

Only the WC powder is applied in the filling process.

FIG. 2 is a photograph taken through a scanning electron microscope ofthe WC powder to be filled. The WC powder has a purity of 99.95% and aparticle size of 0.5 μm. As shown in the photograph, each of particleshas an almost spherical shape, but the particles are agglutinated witheach other.

Further, FIG. 3 shows a result of XRD component analysis of the WCpowder applied to FIG. 2, wherein impurities such as W₂C are notcontained.

As described above, the WC powder to be sintered does not containimpurities other than a WC component.

In the filling process, firstly, the lower punch 216 is inserted into alower portion of the mold 200, the WC powder is filled in the mold 200,the upper punch 215 is inserted into an upper portion of the mold 200,and then a pressure of 1400 to 1600 kgf is preliminarily pressurized for5 to 15 minutes using a molding press so as to increase adhesion forcebetween powder particles.

After the filling process, the installing process in which the mold 200is installed in the chamber 110 of the discharge plasma sinteringapparatus 100 is carried out. At this time, the above-mentioned upperand lower spacers 221, 222, 223, 231, 232 and 233 are installed betweenthe upper and lower electrodes 211 and 212 of the mold 200.

The vacuum-making process is to make a vacuum inside the chamber 110.The inner air of the chamber 110 is discharged by the pump 150, and thusthe vacuum is created in the chamber 110. At this time, an inside of thechamber may be vacuumized to 6 Pa to 1×10⁻³Pa. If the degree of thevacuum in the chamber 110 is too low, the WC powder may be contaminatedand the inside of the chamber 110 may be oxidized by the impurities.

The molding process is to mold the WC powder 205 while applying thecurrent to the WC powder 205. The pressurizer 160 is operated tomaintain a pressure of 30 to 100 MPa, preferably 70 MPa, with respect tothe WC powder 205 in the mold 200, and also the WC power in the mold 200is heated according to a preset heating rate and heating pattern. Atthis time, a final target sintering temperature of the mold 200 may beset to 1410 to 2000° C. If the sintering temperature is less than to1410° C., the molding may be not achieved, or a sintered body having alow density may be prepared. Further, if the sintering temperature isgreater than to 2000° C., particles of the sintered body may suddenlygrow, may be melted, and thus may have a bad influence on mechanicalproperties.

More specifically, in the molding process, the WC powder 205 in the mold200 is primarily heated to a first target temperature of 550 to 650° C.at a heating rate of 60 to 150° C./min. The first target temperature maybe 600° C.

When reaching the first target temperature, the first target temperatureis isothermally maintained for 1 to 10 minutes.

Then, the WC powder 205 in the mold 200 is secondarily heated to asecond target temperature of 900 to 1005° C. at a heating rate of 30 to80° C./min. The second target temperature may be 1000° C.

When reaching the second target temperature, the second targettemperature is isothermally maintained for 1 to 10 minutes.

And then, the WC powder 205 in the mold 200 is thirdly heated to a thirdtarget temperature of 1010 to 1105° C. at a heating rate of 10 to 80°C./min. The third target temperature may be 1100° C.

When reaching the third target temperature, the third target temperatureis isothermally maintained for 1 to 10 minutes.

And then, the WC powder 205 in the mold 200 is fourthly heated to afourth target temperature of 1110 to 1205° C. at a heating rate of 10 to80° C./min. The fourth target temperature may be 1200° C.

When reaching the fourth target temperature, the fourth targettemperature is isothermally maintained for 1 to 10 minutes.

And then, the WC powder 205 in the mold 200 is fifthly heated to a fifthtarget temperature of 1210 to 1305° C. at a heating rate of 10 to 80°C./min. The fifth target temperature may be 1300° C.

When reaching the fifth target temperature, the fifth target temperatureis isothermally maintained for 1 to 10 minutes.

And then, the WC powder 205 in the mold 200 is sixthly heated to a sixthtarget temperature of 1310 to 1405° C. at a heating rate of 10 to 80°C./min. The sixth target temperature may be 1400° C.

When reaching the sixth target temperature, the sixth target temperatureis isothermally maintained for 1 to 10 minutes.

And then, the WC powder 205 in the mold 200 is seventhly heated to aseventh target temperature of 1410 to 2000° C. at a heating rate of 10to 80° C./min. The seventh target temperature may be 1500° C.

When reaching the seventh final target temperature, the seventh targettemperature is isothermally maintained for 1 to 10 minutes.

The heating rate and the applied time in the isothermal state during themolding process are designated in Table 1.

TABLE 1 Target 0 550~650  900~1005 1010~1005 1100~1205 1210~13051310~1405 1410~1600 temp. (° C.) Heating 0 60~150 30~80  10~80 10~8010~80 10~80 10~80 rate. (° C./min) Maintaining 0 1~10 1~10  1~10  1~10 1~10  1~10  1~10 (min)

In the cooling process, after reaching the final target temperature andmaintaining the isothermal state for the applied time, the inside of thechamber 110 is cooled, while a pressure applied to the WC powder 205 inthe mold 200 is maintained.

After the cooling process, a WC sintered body is separated from the mold200. The WC sintered body prepared through the above-mentioned processeshas a structure as illustrated in FIG. 4. In the preparation processes,a high current having a low voltage pulsed phase is introduced into agap between the WC powder particles by the current applied through theupper and lower electrodes 211 and 212, and the sintered body is moldedby thermal diffusion and electro-transport caused by high energy ofdischarge plasma momentarily generated by spark discharge, pressure andheat caused by electric resistance of the mold 200, and electric energy.

Also, the pulsed current activation is a direct heating manner in whichthe current directly flows to the WC as a test piece through the punches215 and 216. Heat is generated in the test piece at the same time whenthe current is applied to the mold 200, and a temperature differencebetween an inside of the test piece and an outside thereof is relativelysmall, and also it is possible to minimize a thermal activation actiongenerated in the sintering process, due to a relative low temperatureand a short sintering time. Particularly, when sintering the WC powder,it is possible to achieve a high density of 99.5% or more and fineparticles which are proper to a friction stir welding tool.

Further, according to the preparation method of the WC sintered body forthe friction stir welding tool, it is possible to prepare a sinteredbody having a large surface area of a diameter of 50 to 150 mm and athickness of 25 to 30 mm.

The WC sintered body can be sintered by only a single process withoutadditional post-treatment processes and a sintering additive such ascobalt, can have the high relative density of 99.5% or more, and alsocan control the fine particles, compared with a conventional sinteringmethod such as pressureless sintering, HP and HIP.

Furthermore, the preparation method according to the present inventioncan prepare the WC sintered body having a diameter and thickness whichis 20 times greater than in the conventional sintering method (thepressureless sintering, the HP and the HIP), and also having uniformphysical properties with high strength, high abrasion resistance andhigh density (high relative density of 99.5% or more), even thoughhaving a greater size.

This is because an isothermal state maintaining interval is providedafter the heating process in order to reduce the temperature differencebetween the inside and the outside as well as the difference in thephysical properties therebetween, and also to prepare the sintered bodyhaving the high density, even though it is faster than the conventionalsintering method.

FIG. 4 shows a WC sintered body which is prepared in accordance with thepresent invention and of which a surface is polished and then etched bya murakami etching process. As shown in FIG. 4, the spherical WC mayhave a plate shape when sintering the WC.

FIGS. 5 and 6 show the sintered bodies prepared by the discharge plasmasintering apparatus before being molded to test pieces having a diameterof 65.5 mm and a thickness of 30 mm.

FIG. 7 is a photograph showing the friction stir welding tool before atest, which is prepared by machining the sintered test piece andinstalled at an FSW apparatus.

FIG. 8 is a photograph of the WC tool of FIG. 5 after installed in theFSW apparatus and performing the friction stir welding test with respectto a SS400 (a tensile strength of 400 MPa class) steel plate. In FIG. 8,stripe type grooves which are deeply dug by frictional movement of theWC tool in a length direction of the SS400 (a tensile strength of 400MPa class) steel plate at a central portion thereof are formed, and itcan be understood from the grooves that the WC tool can be used for thefriction stir welding process.

FIG. 9 is a photograph of the SS400 (a tensile strength of 400 MPaclass) steel plate and a currently available tungsten carbide-cobalttool before and after installed in the FSW apparatus and performing thefriction stir welding test. According to a result of the test which isperformed at a lower level than the test using the WC tool of thepresent invention, it could be understood that the test with respect tothe SS400 (a tensile strength of 400 MPa class) steel plate was notperformed well, and the tungsten carbide-cobalt tool was broken at thesame time when the frictional movement test was begun.

FIG. 10 is a photograph of the WC tool of FIG. 7 after performing thetest using the FSW apparatus. As shown in FIG. 10, compared to thecurrently available tungsten carbide-cobalt tool of FIG. 9, theconventional tool is broken at the same time when the test is begun, orworn out, but in the tool of the sintered body having the relativedensity of 99.8% or more prepared according to the present invention, aprobe or a shoulder portion of the tool is not broken or worn out in thefriction stir welding test of 50 m or more.

FIG. 11 is graph illustrating a change in a weight of the WC tool ofFIG. 7 while performing the friction stir welding test. After thefriction stir welding test of 50 m or more, the change of the weight ofthe WC tool is 0.177 g, and thus it can be understood that the WC toolis hardly worn out.

Meanwhile, FIG. 12 is a graph illustrating a result of XRD componentanalysis with respect to the sintered body prepared by the preparationmethod of the present invention. As shown in FIG. 12, impurities otherthan a WC component are not observed, and W₂C are not absolutely found.

Further, the sintered body prepared by the preparation method of thepresent invention and having a thickness of 30 mm is cut at positionscorresponding to an upper surface and 25 mm, 15 mm and 5 mm from abottom surface, and then hardness and fracture toughness thereof aremeasured, and a result thereof is indicated in Table 2.

TABLE 2 Relative Hardness Fracture toughness Cut position density (%)(kg/mm²) Hv30 (Mpa · m^(1/2)) Surface (30 mm) 99.8 2484.9 4.30 25 mm99.8 2474.6 4.96 15 mm 99.8 2686.4 5.29  5 mm 99.8 2573.4 4.82

Meanwhile, unlike the multi-stage heating pattern indicated in Table 1,when the sintered body is prepared, while being continuously heated tothe final target temperature at a preset heating rate of 30 to 70° C.per one minute, the sintered body has a relative density of about 94%, ahardness of 2200 kg/mm² and a fracture toughness of 6 Mpa·m^(1/2). Fromthe result, when the tool is prepared from a large-sized sintered bodyhaving a diameter of 60 mm or more and a thickness of 30 mm or more, themulti-stage heating pattern indicated in Table 1 may be applied.

Although an exemplary embodiment of the present invention has beendescribed in detail hereinabove, it should be understood that manyvariations and modifications of the basic inventive concept hereindescribed, which may appear to those skilled in the art, will still fallwithin the spirit and scope of the exemplary embodiments of the presentinvention as defined by the appended claims.

What is claimed is:
 1. A preparation method of a tungsten carbidesintered body for a friction stir welding tool, comprising: fillingtungsten carbide (WC) powder in a mold made of a graphite material;installing the mold filled with the WC powder in a chamber of adischarge plasma sintering apparatus; making a vacuum inside thechamber; molding the WC powder in the mold while maintaining a constantpressure inside the mold and increasing a temperature according to a setheating pattern until the temperature reaches a final targettemperature; and cooling the inside of the chamber while maintaining thepressure applied in the mold after the molding.
 2. The preparationmethod of claim 1, wherein the final target temperature of the moldingis 1410 to 2000° C.
 3. The preparation method of claim 2, wherein the WCpowder in the filling has a particle size of 10 to 100 μm, and furthercomprising preliminarily pressurizing the WC powder at a pressure of1400 to 1600 kgf and maintaining the pressure for 5 to 15 minutes, afterthe filling of the WC powder in the mold.
 4. The preparation method ofclaim 3, wherein, in the installing, a plurality of upper spacers madeof a graphite material, of which each diameter is gradually reducedtoward an upper punch, is provided between an upper electrode forapplying an electric field into the mold and the upper punch enteringinto the mold from an upper side of the mold, and a plurality of lowerspacers made of a graphite material, of which each diameter is graduallyreduced toward a lower punch, is provided between a lower electrode inthe chamber and the lower punch entering into the mold from a lower sideof the mold, the upper spacers has a first upper spacer, a second upperspacer and a third upper spacer which are disposed in a circular shapein a direction from the upper electrode to the upper punch, the lowerspacers has a first lower spacer, a second lower spacer and a thirdlower spacer which are disposed in a circular shape in a direction fromthe lower electrode to the lower punch, and the first upper spacer andthe first lower spacer have a diameter of 350 mm and a thickness of 30mm, the second upper spacer and the second lower spacer have a diameterof 300 mm and a thickness of 60 mm, and the third upper spacer and thethird lower spacer have a diameter of 100 to 200 mm and a thickness of15 to 30 mm.
 5. The preparation method of claim 4, wherein, in themaking of the vacuum, the inside of the chamber is vacuumized to 6 Pa to1×10⁻³Pa in order to restrict contamination due to oxidation of the WCpowder and impurities in the chamber, and the molding maintains the WCpowder filled in the mold at a pressure of 30 to 100 MPa.
 6. Thepreparation method of claim 5, wherein the molding comprises, primarilyheating the WC powder in the mold to a first target temperature at aheating rate of 60 to 150° C./min; maintaining the first targettemperature for 1 to 10 minutes; secondarily heating the WC powder inthe mold to a second target temperature at a heating rate of 30 to 80°C./min; maintaining the second target temperature for 1 to 10 minutes;thirdly heating the WC powder in the mold to a third target temperatureat a heating rate of 10 to 80° C./min; maintaining the third targettemperature for 1 to 10 minutes; fourthly heating the WC powder in themold to a fourth target temperature at a heating rate of 10 to 80°C./min; maintaining the fourth target temperature for 1 to 10 minutes;fifthly heating the WC powder in the mold to a fifth target temperatureat a heating rate of 10 to 80° C./min; maintaining the fifth targettemperature for 1 to 10 minutes; sixthly heating the WC powder in themold to a sixth target temperature at a heating rate of 10 to 80°C./min; maintaining the sixth target temperature for 1 to 10 minutes;seventhly heating the WC powder in the mold to a seventh final targettemperature at a heating rate of 10 to 80° C./min; and maintaining theseventh final target temperature for 1 to 10 minutes, wherein the firsttarget temperature is 550 to 650° C., the second target temperature is900 to 1005° C., the third target temperature is 1010 to 1105° C., thefourth target temperature is 1110 to 1205° C., the fifth targettemperature is 1210 to 1305° C., the sixth target temperature is 1310 to1405° C., and the seventh final target temperature is 1410 to 2000° C.